This invention relates generally to multiphase power converters that work in the transition or discontinuous mode. More particularly, the invention relates to maintaining a predetermined phase difference between the phases of the multiphase power converter and operating each phase of the multiphase power converter at its natural frequency in order to increase the efficiency of and reduce the electromagnetic noise produced by the multiphase power converter.
Operating multiple power converter phase circuits in parallel is desirable to improve efficiency, reduce current ripple and increase the frequency of the current ripple. Reduced current ripple together with increased frequency eases the design of the electromagnetic compatibility (i.e., electromagnetic noise) filter, while increased efficiency reduces the cooling requirements of the power converter for a given power conversion level (e.g., smaller heat sinks, running a power converter cooling fan at a lower fan speed, and/or eliminating the need for a power converter cooling fan).
One technique to improve efficiency and electromagnetic compatibility (EMC) behaviour is to employ converter topologies with zero-voltage and zero-current switching. However, quite often those topologies exhibit variable switching frequency, which makes it more difficult to synchronize multiple phases (i.e., operate the phases at a predetermined phase difference such as 180 degrees out of phase). In contrast, fixed switching frequency power converters can be phase controlled simply by establishing a fixed time delay between the time bases. Due to variations in the phase circuits, however, fixed frequency operation may cause a slight increase in electromagnetic noise emissions and non-optimal efficiency if the fixed frequency does not match the natural frequency of all of the phases of the multiphase power converter.
There are open-loop and closed-loop methods known to align the phases (i.e., parallel converters or phase circuits) of variable frequency multiphase power converters. The open-loop methods generate a fixed delay between the phases based on the measurement of the time period of a master phase circuit. However, because the natural switching frequency of each phase circuit is different due to component variances, the slave converters (i.e., slave phase circuits) cannot run at their natural frequencies. The open-loop phase control algorithms override the natural frequency of the slave phases by forcing the slave phases to switch at the same frequency as the master phase. As a result, the open-loop methods may deteriorate the conversion efficiency and EMC behaviour of the multiphase power converter because zero-voltage and zero-current switching conditions may not always be met in the slave phase circuits.
Closed-loop methods adjust the on-time of the slave phase circuits to obtain a fixed phase relationship. Under a closed loop method, all phases are operating at their natural frequencies, maintaining zero-voltage and zero-current switching. Therefore, multiphase power converters using closed loop methods of phase alignment maintain the desirable properties of soft-switching converters such as power conversion efficiency and EMC behaviour, but difficulties arise if one of the converters is running in a clamped frequency mode. To ensure valley switching, the natural frequency of the multiphase power converter can change abruptly due to hopping from one valley to another, which distorts the phase relationship and causes volatility in the input current to the multiphase power converter.